Application of a multicoloring technique by a color conversion method to liquid crystal displays, organic EL displays, lighting apparatuses, and the like is being energetically studied. Color conversion means converting light emitted from a light-emitting body into light having a longer wavelength and means converting blue light emission into green or red light emission, for example.
A composition having this color conversion function is formed into a sheet and combined with, for example, a blue light source, whereby the three primary colors of blue, green, and red can be extracted, that is, white light can be extracted from the blue light source. A white light source obtained by combining the blue light source and the sheet having the color conversion function forms a backlight unit, and this backlight unit, a liquid crystal drive part, and color filters are combined, whereby a full-color display can be produced. Without the liquid crystal drive part, the unit can be used as a white light source as it is, which can be used as the white light source such as LED lighting.
Improvement in color reproducibility is a problem in liquid crystal displays using the color conversion method. Narrowing the half width of the respective emission spectra of blue, green, and red of the backlight unit to increase the color purity of each of blue, green, and red is effective in improving color reproducibility. To solve this problem, developed is a technique that uses quantum dots formed of inorganic semiconductor fine particles as a component of the color conversion material (refer to Patent Literature 1, for example). Although the technique using the quantum dots is indeed narrow in the half width of green and red emission spectra to improve color reproducibility, the quantum dots are vulnerable to heat, and moisture and oxygen in the air and are thus deficient in durability on the other hand.
Also developed is a technique that uses an organic or inorganic luminous material as a component of the color conversion material in place of the quantum dots. Disclosed as examples of the technique that uses an organic luminous material as the component of the color conversion material are one that uses a coumarin derivative (refer to Patent Literature 2, for example), one that uses a rhodamine derivative (refer to Patent Literature 3, for example), and one that uses a pyrromethene derivative (refer to Patent Literature 4, for example).
There is also another problem in that, although using the quantum dots technology and a color conversion material composed of an organic or inorganic luminous material improves color reproducibility, the color characteristics and the light emission characteristics of the color conversion material cause the luminance to decrease. For example, one solution to this problem is disclosed, wherein a light-wavelength-selective reflective film that reflects light emitted from a color conversion material is used (refer to Patent Literature 5, for example).